The δ18O of foraminiferal calcite has long been the standard proxy of ancient ocean temperatures. However, it is confounded by the fact that foraminiferal calcite δ18O, in addition to temperature, is also a function of the δ18O of the ambient seawater. Therefore, it has become common recently to use the Mg/Ca ratio of foraminiferal calcite as a paleothermometer independent of ice volume. Paleotemperatures calculated thus can be applied to the δ18O -paleothermometer to calculate the δ18O of ancient seawater. Unfortunately, in a similar way that the δ18O paleothermometer is impacted by changing water chemistry and temperature, foraminiferal calcite Mg/Ca is a function of ambient seawater Mg/Ca as well as temperature. Attempts to date to employ the Mg/Ca paleothermometer to calculate temperatures and seawater δ18O over long time scales have been hampered by an inaccurate treatment of the effect of seawater Mg/Ca on calcite Mg/Ca. By using a power function, the Mg/Ca paleothermometer can be more accurately applied to deep geologic time (i.e., ages older than 5 Ma). While previous methods assumed little to no change in seawater Mg/Ca over the last 60 My to produce realistic paleocean temperatures and δ18O, the updated methodology presented here reduces the sensitivity of paleotemperature estimates to seawater Mg/Ca. This reconciles the results of Mg/Ca paleothermometry studies with geochemical evidence that propose a large change (from ~1 to ~5) in seawater Mg/Ca since 60 Ma.